Superplasticity is the characteristic demonstrated by certain metals which exhibit extremely high plasticity. They develop high tensile elongations with minimum necking when deformed within specific temperature ranges and limited strain rate ranges. The methods used to form and in some cases diffusion bond superplastic materials capitalize on these characteristic and typically employ gas pressure to form sheet material into or against a configurational die in order to form the part. Diffusion bonding is frequently associated with the process. U.S. Pat. No. 3,340,101 to D. S. Fields, Jr. et al.; U.S. Pat. No. 4,117,970 to Hamilton et al.; U.S. Pat. No. 4,233,829 to Hamilton et al.; and U.S. Pat. No. 4,217,397 to Hayase et al. are all basic patents, with various degrees of complexity, relating to superplastic forming. All of these references teach a process which attempts to control stress, and thereby strain, by controlling the pressure in the forming process versus time.
Exceptions to controlling forming rates by controlling pressure versus time are taught in U.S. Pat. No. 4,708,008 to Yasui et al. and U.S. Pat. No. 5,129,248 to Yasui. Yasui et al. teaches measuring and controlling the volume displaced by the blank being formed so as to measure total strain or surface area increase of the blank while Yasui teaches an apparatus and method for controlling superplastic forming processes by measuring and controlling the gas mass flow rate of the gas displacing the blank being formed.
U.S. Pat. No. 4,489,579 to Daime et al. also teaches controlling the process by controlling pressure versus time, but also teaches additional devices for monitoring the forming rate by providing a tube which penetrates the die and engages a portion of the blank to be formed. As the blank is formed, the tube advances through the die directly as that portion of the blank is formed. Means are also provided to produce a signal at predetermined amounts of advancement of the tube and, further, electrical contacts are provided at recess angles of the die and the switch is closed when the blank being formed, it provides for monitoring the forming step which allows the operator to evaluate the development process of the part. However, it is not very practical to have a sliding rue probe with the associated geometric disturbance at the contact point nor is it practical to provide electrical instrumentation in this harsh environment.
Excessive strain rates cause rupture and must be avoided in the forming process. In order to understand excessive strain rates it is necessary to understand the relationship between the variables in superplastic forming which are represented by the classic equation EQU .sigma.=K.epsilon.m
where m is the strain rate sensitivity, .sigma. is stress, .epsilon.is strain rate, and K is a constant.
In the absence of strain hardening, the higher the value of m, the higher the tensile elongation. Solving the classic equation for m, ##EQU1##
In addition to strain rate, the value of m is also a function of temperature and microstructure of the material. The uniformity of the thinning under biaxial stress conditions also correlates with the value of m. For maximum deformation stability, superplastic forming is optimally performed at or near the strain rate that produces the maximum allowable strain rate sensitivity. However, because the strain rate sensitivity, m, varies with stress as well as temperature and microstructure, m constantly varies during a forming process.
Furthermore, the strain rate varies at different instances of time on different portions of the formation inasmuch as stress levels are non-uniform. The more complex the part, the more variation there is, and, therefore, strain rate differs over the various elements of the formation. Since strain rate, stress, temperature and microstructure are all interdependent and varying during the process, the relationship is theoretical. As a practical matter, there is no predictable relationship that can be controlled so as to form all portions of complex parts at the optimum strain rate sensitivity and therefore the optimum strain rates. However, the artisan can plot strain rate sensitivity (m) against strain rate (.epsilon.) and stress (.sigma.) against strain rate (.epsilon.) and establish the best compromise ranges to be caused as guides. Prior to Yasui, those skilled in the art had to select and control those portions of the formation, which are more critical to successful forming while maintaining all other portions at the best or less than the best strain rates which necessary becomes the overall optimum rate.
This was further complicated for deep forming, which requires forming pressure reduction due to the higher thinning rate of the material, if during the forgoing process, the blank was not be exactly where it is thought to be at any given time in the forming process.
By controlling the process with either pressure or perhaps volume alone, only one of the variables in Boyle's Law ##EQU2## (where P, V, and T represent pressure, volume, and temperature, respectively) was used to control the process. Yasui found that the process was much more stable when instead of controlling pressure which was the accepted practice at the time, the mass of gas used to form was controlled. The stability of this process is due to the recognition that if a controlled mass rate is introduced, when the forming blank is being strained too slowly, the pressure will build up until the applied stress increases to increase the strain rate. When the blank is forming too fast, the pressure drops or at least its rate of increase diminishes to slow down the strain rate due to volume increase. However there has been a need to monitor superplastic forming, or superplastic forming and diffusion bonding processes for early detection of departure from the desired process, so that corrections can be made before the forming part is ruined.